Presently polychromic reproductions are prepared principally using a technique that involves making a mask for each area or group of areas corresponding to a given density for a selected color component. Typically this is done by painting a screen in gouache with a brush to create a screened negative corresponding to the desired density. With this technique one must create similar masks for each density and then assemble the masks of the different densities for each of the selected colors in the polychromatic system on the final positive. It is indispensable using this technique for routing of the successive masks to be carried out with very great precision. If it is not, overlappings which introduce new shades or white edges may be produced which injure the quality of the reproduction. Thus this method is a very tedious one, and requires as many screens as there are densities being used.
Because of the high cost of the screens, methods have been considered that facilitate the creation of intermediate screens with a reduction in the number of screens being the principal objective. However, all of the methods so far employed have finally required as many screened masks as there are areas of a specific density.